Ion Movement Explains Huge V_OC Increase despite Almost Unchanged Internal Quasi‐Fermi‐Level Splitting in Planar Perovskite Solar Cells

Light soaking under “1 sun” is performed on planar p–i–n perovskite solar cells with a Cs0.05MA0.10FA0.85Pb(I0.95Br0.05)3 absorber while measuring current and voltage transients simultaneously to spectral photoluminescence (PL). From theory a tenfold increase in PL intensity is expected for every 60 mV rise in VOC (at 300 K). However, the solar cells investigated show a reversible VOC increase from as low as 0.5 up to 1.05 V during light soaking, whereas the PL intensity hardly changes. A model is developed based on mobile ions in combination with a nonideal contact. It reproduces the decoupling of the VOC and PL as well as the transient behavior in great detail. Using state‐of‐the‐art materials and passivation layers shows that light soaking is still a relevant feature of high‐efficiency perovskite solar cells. The ionic liquid additive 1‐butyl‐3‐methylimidazolium tetrafluoroborate slows down the light‐soaking behavior, giving an example of how ionic motion in perovskite solar cells can be influenced.